[Asawi]

I'm not entirely what classifies as a "classic" but I nominate

Fer-de Lance by Rex Stout (The first Nero Wolfe book)

It is the first in a very long, both in time and numbers, successful detective series.

About the series (from Wikipedia):

Spoiler:

Nero Wolfe is a fictional detective, created in 1934 by the American mystery writer Rex Stout. Wolfe's confidential assistant Archie Goodwin narrates the cases of the detective genius. Stout wrote 33 novels and 39 short stories from 1934 to 1974, with most of them set in New York City. Wolfe's residence, a luxurious brownstone on West 35th Street, features prominently in the series. Many radio, television and film adaptations were made from his works.
The Nero Wolfe corpus was nominated for Best Mystery Series of the Century at Bouchercon 2000, the world's largest mystery convention, and Rex Stout was a nominee for Best Mystery Writer of the Century.